The t(8;21)(q22;q22) translocation, which fuses the ETO gene on human chromosome 8 with the AML1 gene on chromosome 21, is one of the most frequent cytogenetic abnormalities associated with acute myeloid leukemia (AML). It is seen in approximately 12-15 percent of AML cases and is present in about 40 percent of AML with an M2 phenotype. We have generated a murine model of the t(8;21) by retroviral transduction of purified hematopoietic stem cells (HSC). Mice reconstituted with HSC that express AML 1- ETO show distinct developmental abnormalities in the stem cell compartment and within the myeloid lineages. Primitive myeloblasts were increased to approximately 10 percent of bone marrow by ten months post-transplant. Consistent with this observation was a 50-fold increase in myeloid colony forming cells in vitro. Eosinophil myelocytes that exhibited abnormal basophilic granulation were also increased. HSC numbers in the bone marrow of 10-month-old reconstituted animals were 29-fold greater than in transplant-matched control mice, suggesting that AML1-ETO expression overrides the normal genetic control of HSC pool size. In summary, AML1-ETO-expressing animals recapitulate many (and perhaps all) of the developmental abnormalities seen in human patients with the t(8;21), although the animals do not develop leukemia or disseminated disease in peripheral tissues like the liver or spleen. This suggests that secondary mutations are required to progress to acute leukemia. The primary goal of this proposal is to understand the underlying molecular and cellular basis for the developmental dysfunction caused by AML1-ETO expression in HSC. The specific aims will focus on (1), phenotypic and functional characterization of AML1-ETO expressing myeloblasts (2), determining the influence of AML1-ETO on HSC self renewal and identification of candidate self-renewal genes (3), characterization of secondary mutations that cooperate with AML1-ETO to induce AML and (4), functional mapping of domains within ETO that are responsible for the developmental phenotypes observed in HSC and myeloblasts.